For many coating applications such as automotive coatings, aerospace coatings, industrial coatings and architectural coatings, dark colors, such as black and dark blue are particularly desirable for aesthetic purposes. However, dark colored coatings have historically been susceptible to absorption of near-infrared radiation because they often rely on the use of pigments, such as carbon black, that absorb near-infrared radiation in addition to visible radiation. Near-infrared radiation, i.e., light energy having a wavelength of from 700 to 2500 nanometers, constitutes about 45% of the solar energy that reaches the earth's surface. Heat is a direct consequence of the absorption of near-infrared radiation. As a result, dark colored coatings have historically been susceptible to substantially increased temperatures, particularly on sunny days, which is often undesirable for many reasons. Thus, solar heat (near-infrared) reflecting coatings have been desired.
In Principles and formulations for organic coatings with tailored infrared properties, Progress in Organic Coatings, 20:1-25 (1992) (“Brady”) formulation approaches for achieving solar heat reflecting coatings are described. In one approach, a two layer coating system is employed in which an upper layer is colored with pigments that absorb visible radiation but are transparent to near-infrared radiation, such as organic black pigments (perylene blacks are mentioned) or other organic pigments (phthalocyanine blues and greens and carbazole dioxazine violet are identified), and an underlayer, such as a highly reflective white undercoat, that reflects near-infrared radiation, reduces the temperature increase of the coating system. An example of such a coating system is also described in United States Patent Application Publication No. 2004/0191540 A1.
Brady mentions that this approach suffers from some drawbacks. First, according to Brady, the topcoat must be as thin as possible since it will always possess some level of near-infrared absorption. Second, Brady states that the topcoat must have a consistent film thickness for uniform visual appearance, which may not be easy. An additional disadvantage not mentioned by Brady is that the cost and, in some applications, such as aerospace and/or automotive applications, the weight, of a two-layer coating system can be undesirable.
In another approach mentioned by Brady, infrared reflective and infrared transparent pigments are blended in a single coating. A difficulty with this approach, however, has been the ability to match a desired color, particularly dark colors, while achieving improved infrared reflection performance. In other words, the addition of infrared reflective pigments, especially those that are not transparent in the visible wavelength range (from 400 to 700 nanometers), to a coating composition comprising infrared transparent pigments can cause an unacceptable change in the color of the resulting cured coating.
As a result, it would desirable to provide solar reflective coatings and coating systems while still achieving a color similar to a selected color of a coating that is significantly less solar reflective, include attractive dark, including black, colors. The inventions described herein were made in view of the foregoing desire.